1. Field of the Invention
The present invention relates generally to assembling nuclear fuel pellets in multiple enrichment zones and, more particularly, is concerned with a capture row storage tray for holding assembled and collated rows of nuclear fuel pellets without the risk of inadvertently causing pellet displacement or misorientation.
2. Description of the Prior Art
In a typical nuclear reactor, the reactor core includes a large number of elongated fuel assemblies. Conventional designs of these fuel assemblies include top and bottom nozzles with a plurality of elongated transversely spaced guide thimbles extending longitudinally between and connected at opposite ends to the nozzles and a plurality of transverse support grids axially spaced along the guide thimbles. Also, each fuel assembly is composed of a multiplicity of elongated fuel elements or rods transversely spaced apart from one another and from the guide thimbles and supported by the transverse grids between the top and bottom nozzles.
The fuel rods each contain fissile material in the form of a plurality of generally cylindrical nuclear fuel pellets maintained in a row or stack thereof in the rod. The fuel rods are grouped together in an array which is organized so as to provide a neutron flux in the core sufficient to support a high rate of nuclear fission and thus the release of a large amount of energy in the form of heat. A liquid coolant is pumped upwardly through the core in order to extract some of the heat generated in the core for the production of useful work.
Historically, a given fuel rod was filled with nuclear fuel pellets of the same enrichment. More recently, nuclear fuel rods with pellets in multiple zones of different enrichments have been introduced. These new zoned fuel rods contain short lengths of "blanket" pellets at each end. The fuel rods additionally have pellets grouped within from three to five zones of different enrichments between the end zones of blanket pellets. Pellets for filling these enrichment-zoned fuel rods first have to be assembled into the desired sequences of enrichment zones before filling of the fuel rods can commence.
A preferred approach to assembling fuel pellets in the desired sequence of enrichment zones is exemplified by the fuel pellet collating system illustrated and described in the copending patent application cross-referenced above. The fuel pellet collating system includes a tray transfer mechanism adapted to handle and sequentially transfer supply trays of pellets of various enrichments to a pellet collating line. At the collating line, mechanisms are provided for removing pellets from the supply trays, measuring specified lengths of pellets of different enrichments, assembling them into rows of pellets having the specified sequence of enrichment zones, and then loading them onto storage trays. The tray transfer mechanism is also adapted to handle and transfer the storage trays of assembled pellets to a collated storage and transport cabinet so that pellets in the specified sequence of enrichment zones can be subsequently loaded into the fuel rods.
Because the assembled rows of pellets in the specified sequences of enrichment zones will be subsequently transferred from the storage trays in the same sequences into fuel rods, it is important that the order of the pellets in the rows thereof be preserved during transfer of the storage trays from the collating line to the storage and transport cabinet and also during transporting of the trays with the storage and transport cabinet to the location where the pellets will be loaded into fuel rods. Consequently, a need exists for a storage tray construction which will prevent inadvertent displacement or misorientation of the pellets from their desired proper sequences as the trays holding the pellets are handled and moved by mechanized equipment.